A memory cell may be a flash memory cell made of field effect transistors (FETs) that each include a select gate, a floating gate, a drain, and a source. Each memory cell may be read by grounding the source, and applying a voltage to a bitline connected with the drain. By applying a voltage to the wordline connected to the select gate, the cell can be switched xe2x80x9conxe2x80x9d and xe2x80x9coff.xe2x80x9d Each memory cell in an array of memory cells store a xe2x80x9c1xe2x80x9d or a xe2x80x9c0.xe2x80x9d Multilevel cells can store more than a single bit of data.
Programming a cell includes trapping excess electrons in the floating gate to increase the voltage. This reduces the current conducted by the memory cell when the select voltage is applied to the select gate. The memory cell is programmed when the cell current is less than a reference current and the select voltage is applied. The cell is erased when the cell current is greater than the reference current and the select voltage is applied.
A memory array may include multiple pages that are individually accessible. For example, a memory array may contain 64 pages and each page may contain 1 KB of memory cells. Each memory cell may be accessed by making the page that contains the memory cell the active memory page, then accessing the memory cell by selecting the row and column in the memory page that corresponds to the memory cell. The latency of making a memory page active is generally much larger than the access time of a memory cell. For example the page latency may be 3 microseconds while the cell access time may be only 50 nanoseconds.
An error may occur in a memory cell due to internal defects, normal use over a long period of time, non-use for a long period of time, or other factors. Two of the primary data reliability issues for memory cells are the xe2x80x9cdata retentionxe2x80x9d effect and xe2x80x9cread disturbxe2x80x9d effect. The xe2x80x9cdata retentionxe2x80x9d effect is a shift in the stored voltage level toward the erase state that results from the normal passage of time. The xe2x80x9cread disturbxe2x80x9d effect is a shift in the stored voltage level that results from reading the memory cell. For the read disturb effect to be appreciable, many reads must occur. When the stored voltage level shifts too far in either direction, it will be interpreted as representing the next higher or lower voltage level and thus the data will be misread.
An error checking and correction (ECC) circuit detects and optionally corrects errors in a memory array. An ECC circuit typically partitions a memory page into groups of memory cells and checks each group of memory cells independently and then generates a syndrome that indicates which memory cells had errors in each group. For example, a page of memory with 1 KB of memory cells may have 64 groups each containing 16 bytes of memory cells. Based on the algorithm of the ECC circuit and the size of the group, the memory cells in the group can be corrected if the total number of errors in the group is below a threshold. An ECC circuit typically generates a syndrome for each group that indicates which memory cells have errors. A syndrome that contains all zeros indicates that no errors were detected. If the number of errors in a group exceeds the threshold, none of the errors can be repaired. For example, the maximum number of errors that can be corrected in a memory array may be:
Max # of errors=# of pages*# of groups*bits/group xe2x80x83xe2x80x83Eqn. 1
Where:
Max # of errors is the maximum number of errors that can be corrected in the memory array.
# of pages is the number of pages in the memory array.
# of groups is the number of groups per page.
bits/group is the number of bits that can be repaired per group.
For the above example:
Max # of errors=64*64*16
=1KBxe2x80x83xe2x80x83Eqn. 2
The reliability of a memory array is dependent on many factors, some of which are process-dependent. Therefore, it is desirable to periodically test memory arrays to ensure that the manufacturing processes is functioning properly. Currently, complex and expensive test equipment must be connected with the memory array to test the reliability of the memory array. Also, the external test equipment typically cause delays during testing. These delays significantly lengthen the testing period for tests such as xe2x80x9cread disturbxe2x80x9d tests.
An integrated memory self-tester that tests an entire memory array reduces the need for sophisticated external test equipment and reduces the duration of the test. A read test of the memory array can check the memory cells. Optional programmable registers may store the results of the tests. The results may be transmitted from the memory device. The integrated memory self-tester may be initiated via a test signal, be self initiated periodically, or be initiated by other means. The memory array may be tested using one or more of a variety of patterns including all ones, all zeros, or a pattern, such as a checkerboard pattern.